I have combined scanning tunneling microscopy with density functional calculations and Monte Carlo simulations to investigate halogen-induced modifications of Si surfaces, including etching, roughening and step transformations. These investigations have focused on elucidating the atomic-level details associated with the evolution of terrace and step sites on Cl-Si(100)-(2x1) under conditions of halogen supersaturation. I present a novel adsorption mechanism that is accessed upon exposing nearly-saturated Cl-Si(100)-(2x1) to a flux of Cl2, whereby dangling bonds of the nearly-saturated surface mediate the insertion of Cl adsorbates into non-equilibrium adsorption sites. The adsorbates are kinetically trapped within these sites due to energy constraints and lack of available and more favorable adsorption sites. Inserted Cl adsorbates force the surface to evolve along a novel etching pathway at elevated temperature resulting in dimer vacancy formation without the customary Si regrowth features attributed to halogen etching processes. This process is investigated at 700 – 825 K and the reaction mechanism is extracted from analysis of the appropriate rate equations.